Method and apparatus for forming model ice sheets

ABSTRACT

A process is described for rapidly forming upon the surface of a saline solution, a sheet of ice, the rehological properties of which permit the use of the sheet in conjunction with properly scaled models of structures such as offshore oil drilling platforms, ships and other vehicles to predict reliably the full scale behavior of such vehicles or structures during interactions between the structures and natural ice cover. An inert cryogenic fluid is sprayed through finely atomizing nozzles into the region above a pool of saline water, the surface layers of which are maintained at the fluid&#39;&#39;s freezing temperature. The vaporization of the liquid refrigerant is accompanied by the absorption of heat from the surface of the pool. This process is sufficiently violent to cause a relatively homogeneous turbulent flow of expanding cold gas over the pool surface such that the rate of heat transfer to the water surface is significantly enhanced over that which would prevail in free convention heat transfer. The growth of the ice sheet is extremely rapid (e.g. 3 X 10 4 cm./sec.). Consequently, the growth of the individual ice crystals is inhibited in the horizontal direction, and the inclusion of salt is accelerated. The resultant sheet of ice is comprised of extremely small crystals. The structural properties of the ice sheet (elastic modulus and tensile strength) depend upon ambient temperature and salinity of the ice layer (FIG. 1). By controlling growth rate, pool salinity and temperature which is maintained subsequent to freezing, the structural properties of the ice sheet may be varied at will. This sheet of fine crystal ice with variable properties provides an excellent model of full scale ice sheets.

United States Patent Edwards, Jr. et al.

[ Sept. 19, 1972 [54] METHOD AND APPARATUS FOR FORMING MODEL ICE SHEETS [72] Inventors: Roderick Y. Edwards, Jr., Annandale, Va.; David L. Benze, Jessup, Md.

[73] Assignee: Arctec, Incorporated, Bowie, Md.

[22] Filed: July 30, 1971 [21] Appl. No.: 167,783

Primary Examiner-William E. Wayner Attorney-Cushman, Darby & Cushman [5 7] ABSTRACT A process is described for rapidly forming upon the surface of a Saline solution, a sheet of ice, the rehological properties of which permit the use of the sheet in conjunction with properly scaled models of structures LN SPRAY HEADERS LN SUPPLY LINE TOWING CARRIAGE ON I ROUNDWAY RAILS ROOF OF STEEL SHEET BACKED WITH HARDBOARD PERSONNEL ENTRY LOCK such as offshore oil drilling platforms, ships and other vehicles to predict reliably the full scale behavior of such vehicles or structures during interactions between the structures and natural ice cover.

An inert cryogenic fluid is sprayed through finely atomizing nozzles into the region above a pool of saline water, the surface layers of which are maintained at the fluids freezing temperature. The vaporization of the liquid refrigerant is accompanied by the absorption of heat from the surface of the pool. This process is sufficiently violent to cause a relatively homogeneous turbulent flow of expanding cold gas over the pool surface such that the rate of heat transfer to the water surface is significantly enhanced over that which would prevail in free convention heat transfer. The growth of the ice sheet is extremely rapid (e.g. 3 X 10 cm./sec.). Consequently, the growth of the individual ice crystals is inhibited in the horizontal direction, and'the inclusion of salt is accelerated. The resultant sheet of ice is comprised of extremely small crystals. The structural properties of the ice sheet (elastic modulus and tensile strength) depend upon ambient temperature and salinity of the ice layer (FIG. 1). By controlling growth rate, pool salinity and temperature which is maintained subsequent to freezing, the structural properties of the ice sheet may be varied at will. This sheet of fine crystal ice with variable properties provides an excellent model of full scale ice sheets.

36 Claims, 8 Drawing Figures UNDERWATER VIEWPORT PATENTEDSEP 19 m2 SHEET 1 or 5 KOO E mmozD mmmodwz mam 24 IY Z 2 ATTOIN E Y g METHOD AND APPARATUS FOR FORMING MODEL ICE SHEETS BACKGROUND OF THE INVENTION During the design phase of structures and vehi l 5 isting systems. First, the system described herein is which must be operated in ice covered wat rs, it is rapid. All prior techniques known to the present invendesirable to model the behavior of the structures or rS require be e 6 and 2 hou to Produce a vehicles during interaction with the ice cover to predict uniform ice Sheet 1 inch thick. The present system in its the loads which the full scale device must withstand. To preferred embodiment requires 4 hours. Second, this do so requires that the properties of the vehicle-ice 10 system requires no moving parts such as mechanical sheet-fluid system conform to a set of rules which refrigeration or paraffin boilers. The refrigeration porevolve from forming the set of dimensionless diftion of the system requires no maintenance and very litferential equations which apply to the behavior of the tle first cost. Third, due to extremely high ice growth full scale vehicle and the model systems and insuring rates and consequent high solute distribution coeffithat the coefficients of the corresponding terms of the cient (16:1 Salinity of solid formed on solution surface) Salinity of s lution H model and full scale dimensionless differential equavery little salt must be present in the solution in the tions are equal. The requirements which evolve are as tank to form high salinity ice. Ice with high salinity may follows: then have its properties brought within the desired range (one-twentieth to one-hundredth that of full hm=1l vhp (1) scale) by control of the room temperature after freezing. The advantage of requiring low salinity in the pool m=1lMTp (2) is as follows: (a) the fluid is less corrosive, (b) at salinities below 24! the warmer water will fall to the bottom of the tank and it will be possible to call upon the m A thermal energy stored in the subsurface layers to melt I I the ice sheet quite rapidly when the experiment is (pim=1/)\-ip (4) finished. Fourth, the use of cryogenic refrigeration is inexpensive. In late 1970, in the Washington, D.C.-Bal- Lm=1l vLm (5) timore, Md. area of the United States, liquid nitrogen MMMm may be purchased at 16.3 cents/I00 cubic feet which 5 means that a single 1 inch thick ice sheet measuring 60 f =ftj by 8 feet, in plan, will require approximately 67.30 to form. This refrigerant acts also as a dehumidifier, a, (7 reducing further the threat of corrosion within the H room containing the tank. Fifth, the direct contact In: p 40 pressure sprayed system described herein causes the formation of very fine grained ice crystals. The formation of these crystals is caused (a) because of the ex- -VP tremely rapid growth rate and (b) by the presence of a In expressions (1) through (9), the subscripts m and p refer to characteristics of the model and prototype or full scale system respectively; h is ice thickness; 0 is ice strength; E is ice elastic modulus; pi is the mass density of the ice; L is any linear dimension; M is ship mass; I is the mass moment of inertia of the ship (particularly about the midship axis); f is the coefficient of friction of the ice; V is the velocity of the vehicle and A is the ratio of the linear dimension of the full scale vehicle to the linear dimension of the model. Practical models of ships range from one-twentieth scale to one-hundredth scale. In general, the smaller the model the less expensive will be the experiment. Hence, to model full scale ice sheets practically, one must be able to produce ice sheets in a laboratory with a strength, elastic modulus and thickness between one-twentieth and one-hundredth of the strength, elastic modulus and thickness of a full scale ice sheet.

SUMMARY OF THE INVENTION The process described herein may be used to produce model ice sheets with a strength, elastic modulus and thickness between one-twentieth and one-hunmist of microscopic particles of condensed water vapor and liquid nitrogen which fall to the water surface acting as nucleation points for very fine crystals. Extremely fine grained ice is important in model tests since it is desirable to have the crystal size in the model equal to l/k th the full scale ice crystal size.

Other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view, with the ceiling removed, of an ice model test basin utilizing the principles of the invention;

FIG. 2 is a schematic perspective view of the nitrogen distribution system of the test basin of FIG. 1;

FIG. 2A is a cross-sectional view of the circled portion A of FIG. 2, drawn on a larger scale;

FIG. 2B is a cross-sectional view of the circled portion B of FIG. 2 drawn on a larger scale;

FIG. 3 is a schematic perspective view of the temperature sensing system and the pool water circulating system of the test basin;

FIG. 4 is a plot of freezing temperature and temperature of maximum density of the saline solution versus salinity;

FIG. 5 is a plot of ice strength versus salinity; and

FIG. 6 is a composite plot of solute distribution coefficient versus growth rate, and growth rate and liquid nitrogen flow versus room temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of an ice model basin is shown in FIG. 1. The nitrogen supply tank, control valve and temperature sensing system are not shown. The basin consists of a pool, the sides and bottom of which are insulated with efficient moisture resistant insulation such as polyurethane foam. This is necessary to prevent heat flow out of the pool except through the surface. This unidirectional flow of heat best approximates natural conditions. The tank is surrounded by a room with insulated walls. The insulation must be of an efficient moisture-resistant type such as polyurethane. All entrances to the tank which are used frequently during experiments are equipped with double doors or locks.

Suspended from the overhead of the cold room and spanning the surface of the tank is a network of piping (see FIG. 2) which distributes a mist of liquid and gaseous nitrogen in the air space between the roof of the cold room and the pool surface. The roof of the cold room just above the pool is steel plate painted with a flat black paint to approximate a black body and consequently encourage radiant transfer of heat from the pool to this surface which is maintained at a temperature very close to -320F. due to its proximity to the liquid nitrogen distribution system. The nitrogen gas is permitted to leave the cold room through ducts which run under the floor boards of the cold room. In case of anemergency, motor driven blowers in these ducts can clear the room of nitrogen within one minute of shutting down the nitrogen supply valve.

The water in the model basin may be circulated with the auxiliary circulating pump. The ice sheet may be disposed of after completion of a test by breaking up the ice sheet by hand and allowing the circulating system to bring warmer water up from the lower levels of the basin to melt the ice.

FIG. 2 shows the nitrogen distribution system and FIG. 3 the temperature monitoring system. In this particular embodiment of this invention the distribution system is divided into four sections. Each of these sections consists of four spray headers g which are aligned 'with the long axis of the model basin. Each of the four headers g are fed in pairs from a T junction h which is, in turn, fed by a T junction i which is, in turn, fed by one of four sectioninlet lines j. Those four inlet lines to the four sections of the tank each has in it a ball valve d. These valves provide the capability to trim the balance between the four sections of the basin. During the initial freezing runs these valves are adjusted until uniform temperature is obtained along the longitudinal axis of the model basin. The two lines e which feed the four supply lines for the spray header divisions have ball valves 1 installed in them. These valves are fully open, when it is desirable to freeze an ice sheet over the entire surface of the pool. Should it be desirable to use only half of the pool, one or the other valves c may be closed. Should it be desirable to have two distinct ice thicknesses proceeding down the pool, one or the other of these valves may be throttled accordingly, and a temporary baffle placed between the two halves of the ice model basin.

In each of the 16 spray headers 3 there are shown six spray engineering company IIOOM l/4 nozzles e with an orifice diameter of 0.037 inches. These nozzles are of two piece construction and cause the liquid to rotate prior to ejection through the orifice with a resulting hollow cone-shaped finely atomized spray issuing from the nozzle. FIG. 2A shows a nozzle e connected to the header. At each end of each spray header there is a nozzle connected to the top of the header (see details in FIG. 2B). The selection of the orifice size and the number of these gas nozzles is base upon a careful calculation of the amount of gas generated in the lines as a result of the absorption of heat by these cold pipes through the process of radiant and free convection transfer from the pool surface. The liquid nozzles are arranged along the bottom of the headers as shown in FIG. 2. The number of nozzles and nozzle orifice size is base upon the desired liquid nitrogen flow rate, nitrogen supply pressure and vaporization rate in the pipes. With this particular embodiment, a pressure building coil mounted on the nitrogen supply tank (see FIG. 2) maintains the supply pressure between 25 and psig. The nozzles will function at pressures as low as 10 psig. It is possible to treble the liquid nitrogen flow rate obtainable at 10 psig. by using the full I00 psig. available at the supply. Control over the flow rate is exercised by controlling the supply pressure to the headers. This is accomplished by a remote operated pressure regulating valve b. The distribution and orientation of the nozzles is determined by the requirement for uniform coverage of the tank surface by the atomized liquid nitrogen gas. In this particular embodiment the nozzles are aimed along the longitudinal axis of the tank. The nozzle spray axis on any particular header aims in one direction while the nozzles on the adjacent header all aim in the opposite direction.

The ultimate aim of the refrigeration system is to produce a sheet of ice on the pool surface which is uniform in thickness along the tank. Provision has been made to adjust distribution between the four headers. An accurate measure of temperature distribution along the ice-air interface in the pool is obtained by thermocouples 2, 3, 4, 6, 7, 8, 10, ll, 12, I4, 15 and I6 in FIG. 3 and is plotted on a time record by recorder 0 in FIG. 3. The water temperature just under the surface is also'measured and recorded (probes 3, 5, 9 and 13 in FIG. 4). By observing the temperature distribution and adjusting the valves (d in FIG. 2), the operator can insure that uniform distribution is maintained.

The mean temperature in the model basin may be maintained by adjusting the main pressure regulating control valve (b in FIG. 2). This valve is backed up by a quick closing valve (a in FIG. 2) which can quickly cut off the supply of nitrogen to the cold room in case of emergency. A chilled air intake system with a liquid nitrogen dehumidifying heat exchanger is installed. (See FIG. 1). This system is used after the ice sheet has been produced by the direct contact process. The supply to the spray headers is cut down. Liquid nitrogen is supplied to the dehumidifying heat exchanger in the chilled air inlet. Nitrogen gas in the room is removed at a rate required to permit occupation of the cold room by humans, via the ducting under the floor and is replaced with cold fresh air entering via the dehumidifying precooler. The air is tested with an oxygen analyzer to insure that it is breathable. The vent systems are run at low speed, and the room is maintained at the desired temperature by supplying liquid nitrogen at a reduced rate to the spray headers. Terriperatures of 30F. may be maintained in this manner without producing hazardous levels.

The selection of the ice growth rate is base upon a desired value of ice strength. The length of time over which the freezing process is continued is selected on the basis of desired ice thickness. FIG. 5 shows the relationship between ice strength (from in situ cantilever beam tests) and salinity of the ice. Entering this curve with desired ice strength, an ice salinity is selected. Dividing this salinity by the salinity of the tank water (which may also be varied), a value of solute distribution coefficient (S-ice/S-solution) is obtained. Entering the lower curve in FIG. 6 (S-ice/S-solution vs. Growth Rate) results in an appropriate value of growth rate. Using the upper portion of FIG. 6 (Air Temperature vs. Growth Rate), the appropriate value of air temperature for the growth cycle is selected. An estimate of the liquid nitrogen use rate may also be obtained from this portion of FIG. 6. I

The formation of the sheet is started by placing full pressure on the system to bring the room air temperature down rapidly. The temperature is monitored on the recorder (FIG. 3), and the nitrogen control value (b in FIG. 2) is controlled accordingly. Once the desired value of air temperature is attained along the tank, little, if any, adjustment will be necessary to maintain steady temperature. (The freezing process is characterized by relatively steady heat flow over time for ice thickness less than 2 inches). Consequently, automatic control is not necessary.

Generally, when the method is practiced in its preferred form for the formation upon the surface of a saline solution a sheet of ice, the structural properties (elastic modulus and flexural bending strength) of which may be controlled, some preliminary choices must be made among variables including (a) selection of water salinity, (b) selection of desired ice salinity (0) based upon selection of ice salinity, (d) selection of desired ice growth rate, (e) selection of desired room temperature, and (f) selection of desired liquid nitrogen flow rate to attain the desired room temperature. (Note FIGS. 46). The remainder of the preferred practice of the invention includes providing the selected flow of pressurized low boiling liquified gas having a boiling point at atmospheric pressure below 80C., and at a temperature corresponding to a vapor pressure above 10 psig.; controllably dispensing this low boiling liquified gas in liquid and gaseous form into an insulated cold room from a pressurized container; spraying such pressurized liquid from atomizing nozzles and pressurized gas from other of those nozzles at above atmospheric pressure into the chamber containing the pool of saline water; maintaining the temperature desired by control of the flow of the liquified gas into the chamber; upon attaining an ice sheet of the desired thickness, the purging of the compartment with chilled dehumidified fresh air until the compartment is safe for occupancy by a human being suitably protected against cold; and the maintenance of the compartment at desired temperature levels by reduced flow of low boiling liquifled gas to the spray system and by admitting chilled and dehumidified air to the chamber at a rate necessary to produce an atmosphere safe for occupancy of a human being.

It should now be apparent that the method and apparatus for forming model ice sheets as described hereinabove possesses each of the attributes set forth in the specification under the heading Summary of the Invention hereinabove. Because the method and apparatus for forming model ice sheets of the invention can be modified to some extent without departing from the principles of the invention as they have been outlined and explained in this specification, the present invention should be understood as encompassing all such modifications as are within the spirit and scope of the following claims.

What is claimed is:

1. A method for modelling the behavior of a marine structure comprising:

forming an ice sheet upon the surface of a saline water solution contained in a pool in a thermally insulated environment by spraying, from within the environment, toward the surface of the solution, a low boiling liquified gas; and disposing a scale model of the marine structure in the pool and moving the model relative to the ice sheet to produce an interaction of the ice sheet and the model.

2. The method of claim 1 wherein the liquified gas is liquid nitrogen.

3. The method of claim 1 wherein the rate of spraying and the solution concentration are chosen to produce an ice sheet having a strength, an elastic modulus and a thickness between one-twentieth and one-hundredth of the strength, elastic modulus and thickness of a full scale ice sheet and the scale model of the marine structure is scaled to a substantially corresponding fraction of the full size marine structure.

4. The method of claim 3 wherein the liquified gas is sprayed from a plurality of spaced sites at a rate to produce the ice sheet in a substantially uniform thickness at a rate of l X 10' to 4 X 10' centimeters thickness per second upon the pool from the saline water solution therein.

5. The method of claim 2 further comprising maintaining a pressure head on the liquid nitrogen about to be sprayed of 10-100 psig.

6. The method of claim 5 comprising maintaining a pressure head on the liquid nitrogen about to be sprayed of about psig.

7. The method of claim 2 wherein the solution salinity when the spraying step is initiated is between a trace thermally insulated environment of said gas with chilled dehumidified fresh air until the atmosphere of the thermally insulated environment is safe to be breathed by humans suitably protected against coldness.

12. The method of claim 11 further comprising maintaining the atmosphere of the thermally insulated environment cold while conducting the moving step by circulating said low boiling liquified gas through said environment in indirect heat exchange relationship therewith.

' 13. The method of claim 1 comprising the initial step of dividing the pool into a plurality of physically separated segments; and wherein the spraying step comprises spraying the liquified gas from a plurality of spaced sites disposed over each segment of the pool, at differing amounts of liquified gas per unit area per unit time for at least two adjacent segments, to produce ice at a greater rate upon at least one segment than upon at least another segment.

14. The method of claim 1 wherein the flow rate of expanding cold gas over the surface of the pool is sufficiently great as to produce turbulence above the pool surface such that the rate of heat transfer with respect to the pool surface is substantially greater than that which would prevail in free convection heat transfer.

15. The method of claim 1 wherein the force and proximity of the spraying is sufficient to propel particles of liquified gas to the pool surface, there to act as nucleation points for very fine ice crystals.

16. The method of claim 3 wherein the force and proximity of the spraying is sufficient to propel particles of liquified gas to the pool surface, there to act as nucleation points for very fine ice crystals having a size related to the full scale ice sheet in substantially the same ratio as the scale model marine structure bears to the full scale marine structure.

l7. The method of claim 1 wherein the thermally insulated environment is bounded, over the pool, by a black surface facing the pool surface, and the method further comprises maintaining the black surface over the pool at about -320F.

18. A method for forming an ice sheet of controlled elastic modulus and fiexural bending strength upon the surface of a saline water solution of selected salinity contained in a pool in a thermally insulated environment, comprising:

spraying, from a plurality of spaced sites within the environment, toward the surface of the solution, a low boiling liquified gas having a boiling point at atmospheric pressure of below minus 80C. at a sufficient rate to produce ice in a sheet at a rate of l X to 4 X W' -centimeters thickness per second upon the pool from the saline water solution therein.

19. The method of claim 18 further comprising the prior step of disposing a temporary barrier across the pool to divide the pool in two segments with some of said sites disposed over one segment of the pool and the remainder of the sites disposed over the other segment of the pool; and wherein the spraying step consists of spraying more liquified gas per unit area per unit time from said some sites than from said remainder thereof to produce ice at a greater rate upon said one segment than upon said other segment.

20. The method of claim 18 wherein the liquified gas is liquid nitrogen.

21. The method of claim 18 wherein the rate of spraying and the solution concentration are chosen to produce an ice sheet having a strength, an elastic modulus and a thickness between one-twentieth and one-hundredth of the strength, elastic modulus and thickness of a full scale ice sheet and disposing a scale model of the marine structure in the pool and moving the model relative to the ice sheet to produce an interaction of the ice sheet and the model, the

' scale model of the marine structure is scaled to a substantially corresponding fraction of the full size marine structure. I

22. The method of claim 18 further comprising maintaining a pressure head on the liquid nitrogen about to be sprayed of 10-100 psig.

23. The method of claim 20 wherein the solution salinity when the spraying step is initiated is between a trace amount and 35 parts per thousand.

24. The method of claim23 wherein the ratio of ice salinity to solution salinity is between 0.30 and 0.95.

25. The method of claim 18 wherein the How rate of expanding cold gas over the surface of the pool is sufficiently great as to produce turbulence above the pool surface such that the rate of heat transfer with respect to the pool surface is substantially greater than that which would prevail in free convection heat transfer.

26. The method of claim 18 wherein the force and proximity of the spraying is sufficient to propel particles of liquified gas to'the pool surface, there to act as nucleation points for very fine crystals.

27. The method of claim 18 wherein the thermally insulated environment is bounded, over the pool, by a black surface facing the pool surface, and the method further comprises maintaining the black surface over the pool at about 320F.

28. Apparatus for forming model ice sheets comprismg:

wall means defining an enclosed, thermally insulated environment;

wall means defining an upwardly open pool within the enclosed, thermally insulated environment for containing an aqueous solution upon which a sheet of ice is to be formed from the solution;

header and spray nozzle means disposed within the thermally insulated environment over the pool and connected by conduit means to a source of liquified, low boiling gas; the spray nozzle means being aimed to direct a spray of said liquified gas toward said pool.

29. The apparatus of claim 28 wherein the header and spray nozzle means and conduit means are suitable for spraying liquid nitrogen and wherein the conduit means is communicated to a source of liquid nitrogen maintained under a pressure of 10-100 psig.

30. The apparatus of claim 29 wherein the wall means defining the enclosed, thermally insulated environment includes downwardly facing surface means extending over the pool and arranged to accept heat radiated from the pool.

31. The apparatus of claim 30 further including indirect heat transfer means for said downwardly facing surface means for maintaining said downwardly facing surface means at about 3 20F.

32. The apparatus of claim 28 wherein said header means comprise at least two distinct headers, each having at least one said nozzle means thereon; and valving means between each header and said conduit means for permitting adjusting the flow rate of liquified gas issuing from the nozzle means of each header.

33. The apparatus of claim 32 further comprising baffle means dividing said pool into a plurality of separated segments; the location(s) of said baffle means so corresponding to the dispositions of said header means and the nozzles thereof that said valving means may be regulated relative to one another to produce ice sheets of varying properties at varying rates upon the separated pool segments simultaneously.

34. The apparatus of claim 28 further including conduit means for exhausting gas from the thermally insulated environment; means for chilling air outside t hg thermally insulated environment and means for admitting the air so chilled into the thermally insulated environment after ice has been frozen upon said pool, for purging the atmosphere of the thermally insulated environment, to make it possible for humans suitably protected against the cold to work therein.

35. The apparatus of claim 28 wherein the nozzle means further include at least one nozzle for each header communicating from an upper portion of that header for releasing into the thermally insulated environment portions of the liquified gas which have vaporized before being sprayed.

36. The apparatus of claim 28 further including carriage means disposed within the thermally insulated environment for towing models of marine structures through the ice-covered pool. 

1. A method for modelling the behavior of a marine structure comprising: forming an ice sheet upon the surface of a saline water solution contained in a pool in a thermally insulated environment by spraying, from within the environment, toward the surface of the solution, a low boiling liquified gas; and disposing a scale model of the marine structure in the pool and moving the model relative to the ice sheet to produce an interaction of the ice sheet and the model.
 2. The method of claim 1 wherein the liquified gas is liquid nitrogen.
 3. The method of claim 1 wherein the rate of spraying and the solution concentration are chosen to produce an ice sheet having a strength, an elastic modulus and a thickness between one-twentieth and one-hundredth of thE strength, elastic modulus and thickness of a full scale ice sheet and the scale model of the marine structure is scaled to a substantially corresponding fraction of the full size marine structure.
 4. The method of claim 3 wherein the liquified gas is sprayed from a plurality of spaced sites at a rate to produce the ice sheet in a substantially uniform thickness at a rate of 1 X 10 5 to 4 X 10 3 centimeters thickness per second upon the pool from the saline water solution therein.
 5. The method of claim 2 further comprising maintaining a pressure head on the liquid nitrogen about to be sprayed of 10-100 psig.
 6. The method of claim 5 comprising maintaining a pressure head on the liquid nitrogen about to be sprayed of about 100 psig.
 7. The method of claim 2 wherein the solution salinity when the spraying step is initiated is between a trace amount and 35 parts per thousand.
 8. The method of claim 7 wherein the ratio of ice salinity to solution salinity is between 0.30 and 0.95.
 9. The method of claim 1 wherein the scale model marine structure is a scale model offshore well drilling platform.
 10. The method of claim 1 wherein the scale model marine structure is a scale model aquatic vessel hull.
 11. The method of claim 1 comprising the step, intermediate the spraying and moving steps, of purging the thermally insulated environment of said gas with chilled dehumidified fresh air until the atmosphere of the thermally insulated environment is safe to be breathed by humans suitably protected against coldness.
 12. The method of claim 11 further comprising maintaining the atmosphere of the thermally insulated environment cold while conducting the moving step by circulating said low boiling liquified gas through said environment in indirect heat exchange relationship therewith.
 13. The method of claim 1 comprising the initial step of dividing the pool into a plurality of physically separated segments; and wherein the spraying step comprises spraying the liquified gas from a plurality of spaced sites disposed over each segment of the pool, at differing amounts of liquified gas per unit area per unit time for at least two adjacent segments, to produce ice at a greater rate upon at least one segment than upon at least another segment.
 14. The method of claim 1 wherein the flow rate of expanding cold gas over the surface of the pool is sufficiently great as to produce turbulence above the pool surface such that the rate of heat transfer with respect to the pool surface is substantially greater than that which would prevail in free convection heat transfer.
 15. The method of claim 1 wherein the force and proximity of the spraying is sufficient to propel particles of liquified gas to the pool surface, there to act as nucleation points for very fine ice crystals.
 16. The method of claim 3 wherein the force and proximity of the spraying is sufficient to propel particles of liquified gas to the pool surface, there to act as nucleation points for very fine ice crystals having a size related to the full scale ice sheet in substantially the same ratio as the scale model marine structure bears to the full scale marine structure.
 17. The method of claim 1 wherein the thermally insulated environment is bounded, over the pool, by a black surface facing the pool surface, and the method further comprises maintaining the black surface over the pool at about -320*F.
 18. A method for forming an ice sheet of controlled elastic modulus and flexural bending strength upon the surface of a saline water solution of selected salinity contained in a pool in a thermally insulated environment, comprising: spraying, from a plurality of spaced sites within the environment, toward the surface of the solution, a low boiling liquified gas having a boiling point at atmospheric pressure of below minus 80*C. at a sufficient rate to produce ice in a sheet at a rate of 1 X 10 5 to 4 X 10 3 centimeters thickness per second upon the pool from the saline water solution therein.
 19. The method of claim 18 further comprising the prior step of disposing a temporary barrier across the pool to divide the pool in two segments with some of said sites disposed over one segment of the pool and the remainder of the sites disposed over the other segment of the pool; and wherein the spraying step consists of spraying more liquified gas per unit area per unit time from said some sites than from said remainder thereof to produce ice at a greater rate upon said one segment than upon said other segment.
 20. The method of claim 18 wherein the liquified gas is liquid nitrogen.
 21. The method of claim 18 wherein the rate of spraying and the solution concentration are chosen to produce an ice sheet having a strength, an elastic modulus and a thickness between one-twentieth and one-hundredth of the strength, elastic modulus and thickness of a full scale ice sheet and the scale model of the marine structure is scaled to a substantially corresponding fraction of the full size marine structure.
 22. The method of claim 18 further comprising maintaining a pressure head on the liquid nitrogen about to be sprayed of 10-100 psig.
 23. The method of claim 20 wherein the solution salinity when the spraying step is initiated is between a trace amount and 35 parts per thousand.
 24. The method of claim 23 wherein the ratio of ice salinity to solution salinity is between 0.30 and 0.95.
 25. The method of claim 18 wherein the flow rate of expanding cold gas over the surface of the pool is sufficiently great as to produce turbulence above the pool surface such that the rate of heat transfer with respect to the pool surface is substantially greater than that which would prevail in free convection heat transfer.
 26. The method of claim 18 wherein the force and proximity of the spraying is sufficient to propel particles of liquified gas to the pool surface, there to act as nucleation points for very fine crystals.
 27. The method of claim 18 wherein the thermally insulated environment is bounded, over the pool, by a black surface facing the pool surface, and the method further comprises maintaining the black surface over the pool at about -320*F.
 28. Apparatus for forming model ice sheets comprising: wall means defining an enclosed, thermally insulated environment; wall means defining an upwardly open pool within the enclosed, thermally insulated environment for containing an aqueous solution upon which a sheet of ice is to be formed from the solution; header and spray nozzle means disposed within the thermally insulated environment over the pool and connected by conduit means to a source of liquified, low boiling gas; the spray nozzle means being aimed to direct a spray of said liquified gas toward said pool.
 29. The apparatus of claim 28 wherein the header and spray nozzle means and conduit means are suitable for spraying liquid nitrogen and wherein the conduit means is communicated to a source of liquid nitrogen maintained under a pressure of 10-100 psig.
 30. The apparatus of claim 29 wherein the wall means defining the enclosed, thermally insulated environment includes downwardly facing surface means extending over the pool and arranged to accept heat radiated from the pool.
 31. The apparatus of claim 30 further including indirect heat transfer means for said downwardly facing surface means for maintaining said downwardly facing surface means at about -320*F.
 32. The apparatus of claim 28 wherein said header means comprise at least two distinct headers, each having at least one said nozzle means thereon; and valving means between each header and said conduit means for permitting adjusting the flow rate of liquified gas issuing from the nozzle means of each header.
 33. The apparatus of claim 32 further comprising baffle means dividiNg said pool into a plurality of separated segments; the location(s) of said baffle means so corresponding to the dispositions of said header means and the nozzles thereof that said valving means may be regulated relative to one another to produce ice sheets of varying properties at varying rates upon the separated pool segments simultaneously.
 34. The apparatus of claim 28 further including conduit means for exhausting gas from the thermally insulated environment; means for chilling air outside the thermally insulated environment and means for admitting the air so chilled into the thermally insulated environment after ice has been frozen upon said pool, for purging the atmosphere of the thermally insulated environment, to make it possible for humans suitably protected against the cold to work therein.
 35. The apparatus of claim 28 wherein the nozzle means further include at least one nozzle for each header communicating from an upper portion of that header for releasing into the thermally insulated environment portions of the liquified gas which have vaporized before being sprayed.
 36. The apparatus of claim 28 further including carriage means disposed within the thermally insulated environment for towing models of marine structures through the ice-covered pool. 